I have spent ages fault finding this issue and only today tried uninstalling Avast One and hey presto. my NAT network is working!
Basically, I have a Windows VM in Hyper V that uses a Virtual Switch using a NAT network: https://learn.microsoft.com/en-us/virtualization/hyper-v-on-windows/user-guide/setup-nat-network
A few months ago this was working fine, this weekend I came to use it and the internet access on the VM is no longer working. I tried all sorts of things recommended online, but I knew this was working previously so did not think I should have to do anything. Once I uninstalled Avast One everything was working again.
I did notice that the network could not be switched to “trusted” in the Firewall, it would automatically switch back to “untrusted”.
I think I will be going back to using Windows Defender, unless someone can shed some light on this.